This is something I've ignored for awhile, but it keeps cropping up, and occasionally, if the filament snags real good, it can mess up a great print half way through, etc.
Basically I have my filament spools hanging on a PVC pipe mounted directly above the printer, and the printer just sucks it in from the spool, but occasionally it becomes knotted at the spool, or becomes otherwise tangled.
I'm thinking even if I completely unwrapped and rewrapped all of my spools so there was no trace of a tangle, what's to stop it from happening again? -- What's a good strategy for managing this kind of issue?
Edit: From some of the answers below, it seems that maybe the travel of my X carriage back and forth could be why my spools keep getting tangled. -- It looks like many people have their spools at about 90 degrees from mine (rotated relative to the Z axis of the printer), so that the travel of the X carriage won't have that affect. -- Also guides, etc are probably a good idea.
Always keep track of the loose end of the filament. It's either in your extruder, or fixed to the spool (either with a clip or clothespin or just by threading it through the holes which are usually in the rim of the spool, provided for just this purpose). The only way a spool can get tangled is if you let go of the loose end of filament.
Though not technically tangled your filament can also get stuck if the spool does not properly unwind (and the filament gets wrapped around the axle that supports your spool). Make sure your spool can rotate on its holder easily (ball bearings are not needed, a simple round axle is enough). Make sure that your printer pulls on the middle of the spool, and not sideways. If necessary, could build a guide (such as a small ring or tube for the filament to pass through) to keep it centered.